Heat exchanger and process for fabricating same

ABSTRACT

A flat hollow body having a fluid channel inside thereof is made from two plates  2  serving as heat exchanger components, by brazing the plates at the peripheral edge portions thereof. The plate  2  comprises a core layer  21  of aluminum or aluminum alloy, and an Al—Si alloy layer  22  covering each of opposite sides of the core layer  21 . The core layer  21  contains Si diffusing thereinto from the Al—Si alloy layer  22 . The alloy layer  22  has a portion up to 1.65 mass % in Si content. The flat hollow body has high corrosion resistance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application filed under 35 U.S.C. §111 (a)claiming the benefit pursuant to 35 U.S.C. §119(e) (1) of the filingdate of Provisional Application No. 60/465,787 filed Apr. 28, 2003pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to heat exchanges and a process forfabricating the same, and more particularly to heat exchangers suitablefor use in fuel cell systems useful, for example, for fuel cell motorvehicles or cogeneration systems for reducing the CO concentration offuel gas (hydrogen gas) produced by a reformer.

The term “fluoride layer” as used herein and in the appended claimsrefers to a layer made substantially from a fluoride.

BACKGROUND ART

Fuel cell systems are adapted to produce hydrogen gas containing arelatively large amount of CO by a reformer from a lower hydrocarbon gassuch as methane or propane, gasoline, methanol or like fuel, to reducethe CO concentration of the hydrogen gas stepwise by a plurality of heatexchangers to obtain hydrogen gas of high purity and to generateelectric power with the pure hydrogen gas using a fuel cell.

CO diminishing heat exchangers generally in use for fuel cell systemsare those made from stainless steel in view of heat resistance andcorrosion resistance, while it appears feasible to use pure aluminum oran aluminum alloy for making the CO diminishing heat exchanger to bedisposed at the most downstream position in view of the reactiontemperature of about 130 to about 140° C. and in order to ensure a costreduction and a reduction in weight. However, the fuel hydrogen gasproduced by the reformer contains an acid gas component, and the drainwater produced by the CO diminishing heat exchanger has an acidity of 3to 4 in pH, so that the heat exchanger must be given high corrosionresistance by a surface treatment.

A surface treatment process is already known for giving high corrosionresistance to pure aluminum or an aluminum alloy by fluorinating thesurface of the pure aluminum or aluminum alloy material with fluorinegas and thereby forming a passive fluoride film (see, for example, thepublication of JP-A No. 1990-263972, claims).

The CO diminishing heat exchanger of pure aluminum or aluminum alloycomprises a plurality of parallel flat hollow bodies each having a fluidchannel inside thereof, and corrugated aluminum alloy fins interposedbetween respective pairs of adjacent hollow bodies and brazed thereto,each of the flat hollow bodies comprising two plates brazed to eachother at the peripheral edge portions thereof, the two plates beingbulged to define therebetween the fluid channel and header-formingportions communicating with respective opposite ends of the channel.Water containing a long-life coolant flows through the fluid channelsinside the flat hollow bodies, and fuel hydrogen gas produced by areformer flows through clearances between the respective pairs ofadjacent flat hollow bodies.

Such a CO diminishing heat exchanger is fabricated by making plates eachhaving a channel-forming bulging portion and a header-forming bulgingportion extending from each of opposite ends of the channel-formingbulging portion and having a greater width than the bulging portion,from a brazing sheet comprising a core, for example, of JIS A3003 alloyand a cladding made of JIS A4004 alloy brazing material and coveringeach of opposite sides of the core, arranging the plates in superposedpairs each comprising the combination of two plates with openings of thebulging portions of each type opposed to each other in correspondingrelation so that the outer surfaces of bottom walls of theheader-forming bulging portions of the adjacent pairs are in contactwith each other and arranging corrugated fins of bare JIS A3003 alloybetween portions corresponding to the channel-forming bulging portionsof the respective adjacent pairs of plates, and brazing the two platesin each pair to each other along the peripheral edge portions thereof toform a flat hollow body and brazing the corrugated fins to therespective adjacent pairs of flat hollow bodies. It is thought useful toform a fluoride layer over the outer peripheral surfaces of the flathollow bodies and the surfaces of the fins by the process disclosed inthe above publication, i.e., by heating the brazed assembly of the flathollow bodies and the fins in an atmosphere containing fluorinating gas.

However, before the fluoride layer is formed on the heat exchangerassembly obtained by the above fabrication method, an Al—Si alloy layerhaving an Si content of 10 wt. % is formed in the surface layer of outerperiphery of the flat hollow bodies, and this Al—Si alloy layer containsan Al—Si eutectic (Al-12 wt. % Si eutectic). Accordingly, Si reacts withF to form the compound of SiF₄ in the subsequent process offluorination, and this compound evaporates, consequently making itimpossible to uniformly form the fluoride layer of required thicknessover the outer peripheral surfaces of the flat hollow bodies. The flowof the fuel hydrogen gas containing an acid gas component and producedby the reformer through the clearances between the adjacent flat hollowbodies produces drain water of pH 3 to 4. The drain water entails theproblem of causing corrosion to the outer peripheral surfaces of theflat hollow bodies, permitting the corrosion to develop to the JIS A3003alloy forming the core of the hollow bodies. The corrosion of outerperipheral surfaces of the flat hollow bodies develops from crystalgrain boundaries of the remaining Al-12 wt. % Si eutectic.

An object of the present invention is to overcome the foregoing problemand to provide a heat exchanger having high corrosion resistance and aprocess for fabricating the same.

DISCLOSURE OF THE INVENTION

To fulfill the above object, the present invention comprises thefollowing modes.

1) A heat exchanger comprising a heat exchanger component having asurface covered with an Al—Si alloy layer, the Al—Si alloy layer havinga fluoride layer formed in a surface layer portion thereof, the Al—Sialloy layer of the heat exchanger component having a portion up to 1.65mass % in Si content.

2) A heat exchanger set forth in the above para. 1) wherein the fluoridelayer is 2 nm to 10 μm in thickness.

3) A heat exchanger set forth in the above para. 1) wherein the fluoridelayer comprises a fluoride produced by subjecting a surface of the Al—Sialloy layer of the heat exchanger component to a fluorination treatment.

4) A heat exchanger set forth in the above para. 1) wherein an anodicoxide coating is formed over a surface of the Al—Si alloy layer of theheat exchanger component, and a plating layer containing nickel isformed on a surface of the anodic oxide coating, the fluoride layerbeing formed over a surface of the plating layer and comprising afluoride produced by subjecting the surface of the plating layer to afluorination treatment.

5) A heat exchanger set forth in the above para. 1) wherein the fluoridelayer is provided over a surface thereof with at least one superposedlayer group comprising a plating layer containing nickel and a fluoridelayer comprising a fluoride produced by subjecting a surface of theplating layer to a fluorination treatment.

6) A heat exchanger set forth in the above para. 1) wherein the heatexchanger component comprises a core layer of pure aluminum or aluminumalloy, and an Al—Si alloy layer covering each of opposite surfaces ofthe core layer, the core layer containing Si diffused thereinto from theAl—Si alloy layer, the Al—Si alloy layer having a portion up to 1.65mass % in Si content.

7) A heat exchanger set forth in the above para. 6) wherein at least onesurface of the heat exchanger component is exposed to a fluid containingan acid component.

8) A heat exchanger set forth in the above para. 1) wherein the heatexchange component has a portion comprising a core layer of purealuminum or aluminum alloy, and two Al—Si alloy layers coveringrespective opposite surfaces of the core layer, and an intermediatelayer of pure aluminum is formed between one of the Al—Si alloy layersand the core layer, the intermediate layer containing Si diffusedthereinto from the Al—Si alloy layer, the Al—Si alloy layer adjacent tothe intermediate layer having a portion up to 1.65 mass % in Si content.

9) A heat exchanger set forth in the above para. 1) wherein the heatexchange component comprises a core layer of pure aluminum or aluminumalloy, and two Al—Si alloy layers covering respective opposite surfacesof the core layer, and an intermediate layer of pure aluminum is formedbetween each of the Al—Si alloy layers and the core layer, theintermediate layer containing Si diffused thereinto from the Al—Si alloylayer, the Al—Si alloy layer having a portion up to 1.65 mass % in Sicontent.

10) A heat exchanger set forth in the above para. 8) or 9) wherein thepure aluminum making the intermediate layer has added thereto Zr and/orMg in a total amount of 0.1 to 0.25 mass %.

11) A heat exchanger set forth in the above para. 8) or 9) wherein theintermediate layer has a thickness in a proportion of 5 to 25% of theentire thickness taken as 100% of the heat exchanger component.

12) A heat exchanger set forth in the above para. 8) or 9) wherein asurface of the heat exchanger component on the side thereof where theintermediate layer exists is exposed to a fluid containing an acidcomponent.

13) A heat exchanger set forth in the above para. 1) which comprises aplurality of parallel hollow bodies each having a fluid channel insidethereof and fins arranged between and brazed to respective pairs ofadjacent flat hollow bodies, the heat exchanger component being each ofthe flat hollow bodies.

14) A heat exchanger set forth in the above para. 1) which comprises aplurality of parallel hollow bodies each having a fluid channel insidethereof and fins arranged between and brazed to respective pairs ofadjacent flat hollow bodies, each of the hollow bodies comprising twoplates brazed to each other at peripheral edge portions thereof, the twoplates defining therebetween a bulging fluid channel and a bulgingheader-forming portion communicating with each of opposite ends of thefluid channel, the heat exchanger component being each of the plates.

15) A heat exchanger set forth in the above para. 13) or 14) wherein afluid containing an acid component flows through at least one of thefluid channel inside each of the flat hollow bodies and a clearancebetween each pair of adjacent flat hollow bodies.

16) A heat exchanger set forth in the above para. 13) or 14) whereinfuel hydrogen gas produced by reforming in a fuel cell system flowsthrough a clearance between each pair of adjacent flat hollow bodies,and an outer peripheral surface of each of the flat hollow bodies iscovered with an Al—Si alloy layer, a fluoride layer being formed in asurface layer portion of the Al—Si alloy layer, the Al—Si alloy layerhaving a portion up to 1.65 mass % in Si content, a catalyst forselectively oxidizing CO being provided on the outer peripheral surfaceof each of the flat hollow bodies and on a surface of each of the fins,the catalyst being serviceable to diminish CO in the fuel hydrogen gas.

17) A fuel cell system comprising a heat exchanger set forth in any oneof the above para. 1) to 16) for diminishing CO.

18) A fuel cell motor vehicle having installed therein a fuel cellsystem set forth in the above para. 17).

19) A cogeneration system comprising a fuel cell system set forth in theabove para. 17).

20) A process for fabricating a heat exchanger characterized by makingplates each having a channel-forming bulging portion and aheader-forming bulging portion bulging to a greater extent than thebulging portion and extending from each of opposite ends of thechannel-forming bulging portion, from a brazing sheet comprising a coreof pure aluminum or aluminum alloy and a cladding of Al-7.5-12.5 wt. %Si alloy brazing material covering each of opposite sides of the core,arranging the plates in superposed pairs each comprising the combinationof two plates with openings of the bulging portions of each type opposedto each other in corresponding relation so that outer surfaces of bottomwalls of the header-forming bulging portions of the adjacent pairs arein contact with each other and arranging fins of bare pure aluminum oraluminum alloy between portions corresponding to the channel-formingbulging portions of the respective adjacent pairs of plates, preheatingthe resulting combination of the pairs of plates and the fins to diffusethe Si in the cladding of the brazing sheet providing the plates throughthe core, brazing the two preheated plates in each pair to each otheralong the peripheral edge portions thereof to form a flat hollow body,brazing the fins to the respective adjacent pairs of flat hollow bodies,and heating the brazed assembly of the flat hollow bodies and the finsin an atmosphere containing a fluorinating gas to form a fluoride layerover surfaces of the flat hollow bodies and surfaces of the fins.

21) A process for fabricating a heat exchanger characterized by makingplates each having a channel-forming bulging portion and aheader-forming bulging portion bulging to a greater extent than thebulging portion and extending from each of opposite ends of thechannel-forming bulging portion, from a brazing sheet comprising a coreof pure aluminum or aluminum alloy, a cladding of Al-7.5-12.5 wt. % Sialloy brazing material covering each of opposite sides of the core, andan intermediate layer of pure aluminum formed between the core and thecladding over at least one of the opposite sides thereof, arranging theplates in superposed pairs each comprising the combination of two plateswith openings of the bulging portions of each type opposed to each otherin corresponding relation so that outer surfaces of bottom walls of theheader-forming bulging portions of the adjacent pairs are in contactwith each other and arranging fins of bare pure aluminum or aluminumalloy between portions corresponding to the channel-forming bulgingportions of the respective adjacent pairs of plates, preheating theresulting combination of the pairs of plates and the fins to diffuse theSi in the cladding of the brazing sheet providing the plates through thecore, brazing the two preheated plates in each pair to each other alongthe peripheral edge portions thereof to form a flat hollow body, brazingthe fins to the respective adjacent pairs of flat hollow bodies, andheating the brazed assembly of the flat hollow bodies and the fins in anatmosphere containing a fluorinating gas to form a fluoride layer over asurface of each of the flat hollow bodies on the core side thereof wherethe intermediate layer exists and over surfaces of the fins.

22) A process for fabricating a heat exchanger set forth in the abovepara. 21) wherein the pure aluminum providing the intermediate layer ofthe brazing sheet making the plates has added thereto Zr and/or Mg in atotal amount of 0.1 to 0.25 mass %.

23) A process for fabricating a heat exchanger set forth in the abovepara. 21) wherein the intermediate layer of the brazing sheet providingthe plates has a thickness in a proportion of 5 to 25% of the entirethickness taken as 100% of the brazing sheet.

24) A process for fabricating a heat exchanger set forth in the abovepara. 20) or 21) wherein the core of the brazing sheet providing theplates and the fins are each made of JIS A3003 alloy.

25) A process for fabricating a heat exchanger set forth in the abovepara. 20) or 21) wherein the cladding of the brazing sheet providing theplates has a thickness in a proportion of 2 to 25% of the entirethickness taken as 100% of the brazing sheet.

26) A process for fabricating a heat exchanger set forth in the abovepara. 20) or 21) wherein the fluorinating gas is at least one gasselected from the group consisting of fluorine gas, chlorine trifluoridegas and nitrogen fluoride gas, and the fluorinating gas is diluted withan inert gas to prepare the atmosphere.

27) A process for fabricating a heat exchanger set forth in the abovepara. 26) wherein the atmosphere contains the fluorinating gas at aconcentration of 5 to 80%.

28) A process for fabricating a heat exchanger set forth in the abovepara. 26) wherein the atmosphere contains the fluorinating gas at aconcentration of 10 to 60%.

29) A process for fabricating a heat exchanger set forth in the above20) or 21) wherein a catalyst for selectively oxidizing CO is providedon outer peripheral surfaces of the flat hollow bodies and on surfacesof the fins after the fluoride layer is formed.

30) A product of pure aluminum or aluminum alloy comprising a componenthaving a surface covered with an Al—Si alloy layer, the Al—Si alloylayer having a fluoride layer formed on a surface layer portion thereof,the Al—Si alloy layer of the component having a portion up to 1.65 mass% in Si content.

31) A product of pure aluminum or aluminum alloy in the above-para. 30)wherein the fluoride layer is 2 nm to 10 μm in thickness.

32) A product of pure aluminum or aluminum alloy in the above para. 30)wherein the fluoride layer comprises a fluoride produced by subjecting asurface of the Al—Si alloy layer of the component to a fluorinationtreatment.

33) A product of pure aluminum or aluminum alloy set forth in the abovepara. 30) wherein an anodic oxide coating is formed over a surface ofthe Al—Si alloy layer of the component, and a plating layer containingnickel is formed over a surface of the anodic oxide coating, thefluoride layer being formed over a surface of the plating layer andcomprising a fluoride produced by subjecting the surface of the platinglayer to a fluorination treatment.

34) A product of pure aluminum or aluminum alloy in the above para. 30)wherein the fluoride layer is provided over a surface thereof with atleast one superposed layer group comprising a plating layer containingnickel and a fluoride layer comprising a fluoride produced by subjectinga surface of the plating layer to a fluorination treatment.

35) A product of pure aluminum or aluminum alloy in the above para. 30)wherein the component comprises a core layer of pure aluminum oraluminum alloy, and an Al—Si alloy layer covering each of oppositesurfaces of the core layer, the core layer containing Si diffusedthereinto from the Al—Si alloy layer, the Al—Si alloy layer having aportion up to 1.65 mass % in Si content.

36) A product of pure aluminum or aluminum alloy in the above para. 35)wherein at least one surface of the component is exposed to a fluidcontaining an acid component or alkaline component.

37) A product of pure aluminum or aluminum alloy set forth in the abovepara. 30) wherein the component has a portion comprising a core layer ofpure aluminum or aluminum alloy, and two Al—Si alloy layers coveringrespective opposite surfaces of the core layer, and an intermediatelayer of pure aluminum is formed between one of the Al—Si alloy layersand the core layer, the intermediate layer containing Si diffusedthereinto from the Al—Si alloy layer, the Al—Si alloy layer adjacent tothe intermediate layer having a portion up to 1.65 mass % in Si content.

38) A product of pure aluminum or aluminum alloy set forth in the abovepara. 30) wherein the component comprises a core layer of pure aluminumor aluminum alloy, and two Al—Si alloy layers covering respectiveopposite surfaces of the core layer, and an intermediate layer of purealuminum is formed between each of the Al—Si alloy layers and the corelayer, the intermediate layer containing Si diffused thereinto from theAl—Si alloy layer, the Al—Si alloy layer having a portion up to 1.65mass % in Si content.

39) A product of pure aluminum or aluminum alloy set forth in the abovepara. 37) or 38) wherein the pure aluminum making the intermediate layerhas added thereto Zr and/or Mg in a total amount of 0.1 to 0.25 mass %.

40) A product of pure aluminum or aluminum alloy set forth in the abovepara. 37) or 38) wherein the intermediate layer has a thickness in aproportion of 5 to 25% of the entire thickness taken as 100% of thecomponent.

41) A product of pure aluminum or aluminum alloy set forth in the abovepara. 37) or 38) wherein a surface of the component on the side thereofwhere the intermediate layer exists is exposed to a fluid containing anacid component or alkaline component.

The present invention includes the following modes.

a) A brazing sheet for use in fabricating heat exchangers whichcomprises a core of pure aluminum or aluminum alloy, two claddings ofAl-7.5-12.5 mass % Si alloy covering respective opposite surfaces of thecore, and an intermediate layer of pure aluminum formed between one ofthe claddings and the core.

b) A brazing sheet for use in fabricating heat exchangers which is setforth in the above para. a) and wherein the pure aluminum making theintermediate layer has added thereto Zr and/or Mg in a total amount of0.1 to 0.25 mass %.

c) A brazing sheet for use in fabricating heat exchangers which is setforth in the above para. a) or b) and wherein the intermediate layer hasa thickness in a proportion of 5 to 25% of the entire thickness taken as100% of the brazing sheet.

d) A brazing sheet for use in fabricating heat exchangers which is setforth in any of the above para. a) to c) and wherein the core is made ofJIS A3003 alloy.

e) A brazing sheet for use in fabricating heat exchangers which is setforth in any one of the above para. a) to d) and wherein the claddinghas a thickness in a proportion of 2 to 25% of the entire thicknesstaken as 100% of the brazing sheet.

With the heat exchanger set forth in the above para. 1), the Al—Si alloylayer of the heat exchanger component has a portion up to 1.65 mass % inSi content. Accordingly, the Al—Si alloy layer is almost unlikely tocontain Al-12 mass % Si eutectic, SiF₄ is inhibited in conducting afluorination treatment for forming the fluoride layer, and the fluoridelayer can be formed uniformly with a required thickness. The heatexchanger component is therefore given high corrosion resistance.Further because Al-12 mass % Si eutectic is almost unlikely to exist inthe Al—Si alloy layer, development of corrosion from crystal grainboundaries of the eutectic to the core layer which is positionedinwardly of the Al—Si alloy layer can be prevented even if the componentis exposed to a fluid having an acid component in the case where thefluoride layer has faults.

The heat exchanger described in the above para. 2) is available at areduced cost and yet has high corrosion resistance against acids.

With the heat exchanger described in the above para. 4) or 5), the heatexchanger component is further improved in corrosion resistance againstacids.

With the heat exchanger set forth in the above para. 6), the Si of theAl—Si alloy-layer is diffused into the core layer, consequently givingan Si content of up to 1.65 mass % to a major portion of the Al—Si alloylayer and permitting the heat exchanger to have the same advantage asthe heat exchanger of the para. 1).

With the heat exchanger set forth in the above para. 7), corrosion isinhibited even when the heat exchanger component is exposed to a fluidhaving an acid component.

With the heat exchanger set forth in the above para. 8) or 9), the Si ofthe Al—Si alloy layer is diffused into the intermediate layer, therebygiving an Si content of up to 1.65 mass % to a major portion of theAl—Si alloy layer and permitting the heat exchanger to exhibit the sameadvantage as the heat exchanger of the para. 1).

With the heat exchanger set forth in the above para. 10), theintermediate layer has pure aluminum crystal grains of increased sizes,and Al-12 mass % Si eutectic is no longer produced even in the layerinto which Si diffuses from the Al—Si alloy layer. This results inimproved corrosion resistance.

With the heat exchangers set forth in the para. 13) to 15), a fluidcontaining an acid component will flow through at least one of the fluidchannel inside each flat hollow body and the clearance between each pairof adjacent flat hollow bodies. The surfaces of the flat hollow bodiesto be exposed to the fluid in this case are covered with the Al—Si alloylayer a major portion of which is up to 1.65 mass % in Si content, and afluoride layer is formed in the surface layer portion of the Al—Si alloylayer. This structure prevents the corrosion of the hollow bodies thatwould result from exposure to the fluid having an acid component.

With the heat exchanger described in the above para. 16), the fuelhydrogen gas produced by reforming and containing an acid gas is passedthrough the exchanger. The surfaces of the flat hollow bodies to beexposed to the fuel hydrogen gas are covered with the Al—Si alloy layera major portion of which is up to 1.65 in Si content, and the fluoridelayer is formed in a surface layer portion of the Al—Si alloy layer. Theflat hollow bodies are prevented from corroding owing to the fluidhaving an acid component.

With the process set forth in the above para. 20) for fabricating a heatexchanger, preheating of the assembly to be brazed causes the Si in thecladding of the brazing sheet providing the plates to diffuse into thecore. Accordingly, the cladding is unlikely to contain Al-12 mass % Sieutectic, SiF₄ is inhibited in conducting a fluorination treatment forforming the fluoride layer after brazing, and the fluoride layer can beformed uniformly with a required thickness. Further because Al-12 mass %Si eutectic is unlikely to exist in the cladding, development ofcorrosion from crystal grain boundaries of Al-12 mass % Si eutectic tothe cladding can be prevented even if the heat exchanger is exposed to afluid having an acid component in the case where the fluoride layer hasfaults.

With the process set forth in the above para. 21) for fabricating a heatexchanger, the Si in the cladding diffuses into the intermediate layerduring brazing. Accordingly, the cladding is unlikely to contain Al-12mass % Si eutectic, SiF₄ is inhibited in conducting a fluorinationtreatment for forming the fluoride layer after brazing, and the fluoridelayer can be formed uniformly with a required thickness. Further becauseAl-12 mass % Si eutectic is unlikely to exist in the cladding,development of corrosion from crystal grain boundaries of Al-12 mass %Si eutectic to the intermediate layer and to the cladding can beprevented even if the heat exchanger is exposed to a fluid having anacid component in the case where the fluoride layer has faults.

With the process set forth in the above para. 22) for fabricating a heatexchanger, the intermediate layer has pure aluminum crystal grains ofincreased sizes, and Al-12 mass % Si eutectic is no longer produced evenin a diffused layer which is formed by the diffusion of Si from thecladding into the intermediate layer. This results in improved corrosionresistance.

With the process set forth in the above para. 27) or 28) for fabricatinga heat exchanger, the exchanger is available at a reduced productioncost, and the fluoride layer of required thickness can be formedrelatively promptly.

The pure aluminum or aluminum alloy product described in the para. 30)has the same advantage as the heat exchanger of the para. 1).

The pure aluminum or aluminum alloy product described in the para. 31)has the same advantage as the heat exchanger of the para. 2).

The pure aluminum or aluminum alloy products described in the para. 33)to 39) have the same advantage as the heat exchanger of the para. 4).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an embodiment of heat exchanger ofthe invention.

FIG. 2 is a fragmentary perspective view partly broken away and showingthe heat exchanger of FIG. 1 on an enlarged scale.

FIG. 3 is an enlarged sectional view showing a plate for making a flathollow body of the heat exchanger of FIG. 1.

FIG. 4 is an enlarged sectional view showing a modified plate for makingthe flat hollow body of the heat exchanger of FIG. 1. FIG. 5 is anenlarged sectional view showing another modified plate for making theflat hollow body of the heat exchanger of FIG. 1.

FIG. 6 is an enlarged sectional view showing a still another modifiedplate for making the flat hollow body of the heat exchanger of FIG. 1.

FIG. 7 is a photograph showing the result of Experimental Example 1.

FIG. 8 is a graph showing the results of Experimental Examples 6 and 7and Comparative Experimental Examples 3 to 5.

FIG. 9 is a photograph showing the result of Experimental Example 8.

BEST MODE OF CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below withreference to the drawings.

Incidentally in the following description, the upper, lower sides andleft- and right-hand sides of FIG. 1 will be referred to as “upper,”“lower,” “left” and “right,” respectively.

FIG. 1 shows the overall construction of a heat exchanger embodying theinvention, and FIGS. 2 and 2 are fragmentary views of the same.

With reference to FIGS. 1 and 2, a heat exchanger 1 comprises aplurality of parallel flat hollow bodies 5 each composed of two dishlikeplates 2 brazed to each other at their peripheral edge portions anddefining therebetween a bulging fluid channel 3 and two bulgingheader-forming portions 4 extending respectively from opposite left andright ends of the channel 3.

The fluid channel 3 and the bulging header-forming portions 4 are formedby arranging two plates 2, each having a channel-forming bulging portion2 a and two header-forming bulging portions 2 b extending fromrespective opposite ends of the portion 2 a, as opposed to each other,with the openings of these portions 2 a, 2 b of one of the plates 2facing toward like openings of the other plate 2 in correspondingrelation. The header-forming portions 4 of the flat hollow body 5 have aheight larger than the height of the fluid channel 3 thereof, and thecorresponding header-forming portions 4 of the adjacent flat hollowbodies 5 are in communication with each other to provide a header 6 ateach of the left and right ends of the exchanger. The fluid channel 3has disposed therein an inner corrugated fin 7 of bare material of purealuminum or aluminum alloy and brazed to the two plates 2. A clearancebetween portions corresponding to the fluid channels 3 of the adjacentflat hollow bodies 5 serves as a gas-phase fluid channel 8 havingdisposed therein an outer corrugated fin 9 made of bare material of purealuminum or aluminum alloy and brazed to the hollow bodies 5. A sideplate 10 of bare material of pure aluminum or aluminum alloy is disposedexternally of and spaced apart from a portion of the flat hollow body ateach of upper and lower ends of the heat exchanger which portioncorresponds to the fluid channel 3 thereof. The space between the sideplate 10 and the hollow body 5 at each of upper and lower ends servesalso as a gas-phase fluid channel 8. An outer corrugated fin 9 isprovided also in this gas-phase fluid channel 8 and brazed to the hollowbody 5 and the side plate 10. The side plate 10 has its opposite endsvertically bent inward and brazed to the header-forming portions 4 ofthe end hollow body 5. A fluid inlet pipe 11 is joined to an upper endportion of the header 6 at the left, and a fluid outlet pipe 12 to alower end portion of the header 6 at the right. The fluid flowing intothe header 6 at the left through the inlet pipe 11 dividedly flowsthrough the fluid channels 3 of all the flat hollow bodies 5 into theheader 6 at the right, and is sent out of the outlet pipe 12.

According to the present embodiment, the plate 2 making the flat hollowbody 5 is a heat exchange component of the invention. With reference toFIG. 3, the plate 2 comprises a core layer 21 of pure aluminum oraluminum alloy, i.e., JIS A3003 alloy according to the presentembodiment, an Al—Si alloy layer 22 covering each of opposite surfacesof the core layer 21, and a fluoride layer 23 formed in a surface layerportion of the alloy layer 22. The material for forming the core layer21 is not limited to JIS A3003 alloy.

Si is diffused from the Al—Si alloy layer 22 into the core layer 21. Thediffused layer is indicated at 24. A major portion of the entire Al—Sialloy layer 22 is up to 1.65 mass % in Si content. In the portion of theplate 2 having a fillet formed by brazing the corrugated fins 7, 9thereto, or the portion of the plate 2 where the plate is bent, the Sicontent of the Al—Si alloy layer 22 may be in excess of 1.65 mass %,whereas the Al—Si alloy layer 22 in the other portion is up to 1.65 mass% in Si content. When a major portion of the entire Al—Si alloy layer 22is up to 1.65 mass % in Si content, Al-12 mass % Si eutectic is unlikelyto exist in the Al—Si alloy layer 22. Consequently, the fluoride layer23 can be formed uniformly with a required thickness by a fluorinationtreatment, with SiF₄ inhibited. Further because Al-12 mass % Si eutecticis unlikely to exist in the Al—Si alloy layer 22, development ofcorrosion from crystal grain boundaries of the eutectic to the corelayer 21 can be prevented even if a fluid having an acid component flowsthrough the fluid channels 3 or gas-phase fluid channels 8 in the casewhere the fluoride layer 23 has faults.

The fluoride layer 23 comprises a fluoride produced by subjecting thesurface of the Al—Si alloy layer 22 of the plate 2 to the fluorinationtreatment. Preferably, the fluoride layer 23 is 2 nm to 10 μm inthickness. If the fluoride layer 23 is less than 2 nm in thickness,sufficient corrosion resistance is not available against acids,permitting the plate to develop corrosion within a relatively shortperiod of time. If the thickness is in excess of 10 μm, formation of thefluoride layer 23 requires much time to increase the production cost ofthe heat exchanger although satisfactory corrosion resistance isavailable. It is desired that the fluoride layer be 20 nm to 3 μm inthickness.

The heat exchanger 1 is used in a fuel cell system, for example, for usein fuel cell motor vehicles or cogeneration systems as a CO diminishingheat exchanger to reduce the CO concentration of the fuel hydrogen gasproduced by a reformer. In this case, a catalyst (not shown) forselectively oxidizing CO is provided on the outer peripheral surfaces ofthe flat hollow bodies, i.e., on the outer surfaces of the plates 2 andthe surfaces of the outer corrugated fins 9. This catalyst may beadhered as supported on a carrier to the outer surfaces of the plates 2and the surfaces of the corrugated fins 9. Although not limitative, thecatalyst to be used is, for example, a Cu—Zn catalyst or zeolitecatalyst. The reaction of CO+1/2O₂→CO₂ is promoted by the catalyst toreduce the CO concentration of fuel hydrogen gas.

When the heat exchanger 1 is used for diminishing CO in fuel hydrogengas, the fuel hydrogen gas is passed through the gas-phase fluidchannels 8 (see FIG. 1, arrow A) and has its CO concentrationcatalytically reduced while being cooled with a refrigerant, e.g., wateror water containing a long-life coolant, flowing through the fluidchannels 8.

When the heat exchanger is used for diminishing CO, the Al—Si alloylayer 22 on the inner peripheral surfaces of the flat hollow bodies 5,i.e., on the inner surfaces of the plates 2, need not always be up to1.65 mass % in Si content, nor is it necessary to provide the fluoridelayer 23 in the surface layer portion of the alloy layer because thecorrosion resistance of the Al—Si alloy layer has no problem when thelayer is exposed to water or water containing a long-life coolant.

When a fluid containing an acid component is passed through both thefluid channels 3 and the gas-phase fluid channels 8, a major portion ofthe entire Al—Si alloy layer 22 on the inner and outer peripheralsurfaces of the flat hollow bodies, i.e., the inner and outer surfacesof the plates 2, is given an Si content of up to 1.65 mass %, and afluoride layer 23 is formed in the surface layer portion of the Al—Sialloy layer 22.

The heat exchanger 1 is fabricated by the process to be described below.

First, a three-layer brazing sheet is prepared which comprises a core ofpure aluminum or aluminum alloy, i.e., JIS A3003 alloy according to thepresent embodiment, and a cladding covering each of opposite sides ofthe core and made of Al-7.5-12.5 mass % Si alloy brazing material, i.e.,JIS A4004 alloy brazing material according to the embodiment. Plates 2are prepared from the brazing sheet by press work, each of the plates 2having a channel-forming bulging portion 2 a and a header-formingbulging portion 2 b bulging to a greater extent than the bulging portion2 a and extending from each of opposite ends of the bulging portion 2 a.Preferably, the cladding of the brazing sheet has a thickness in aproportion of 2 to 25% of the entire thickness taken as 100% of thebrazing sheet because if the thickness is outside this range, difficultyis encountered in producing the brazing sheet by rolling.

The plates 2 are then arranged in superposed pairs each comprising thecombination of two plates 2 with openings of the bulging portions 2 a (2b) of each type opposed to each other in corresponding relation so thatthe outer surfaces of bottom walls of the header-forming bulgingportions 2 b of the adjacent pairs are in contact with each other. Outercorrugated fins 9 of bare pure aluminum or aluminum alloy, i.e., JISA3003 alloy according to the present embodiment, are arranged betweenportions corresponding to the channel-forming bulging portions 2 a ofthe respective adjacent pairs of plates 2, and inner corrugated fins 7of bare pure aluminum or aluminum alloy, i.e., JIS A3003 alloy accordingto the present embodiment, are arranged in the channel-forming bulgingportions 2 a of the respective pairs of plates 2.

The resulting combination of the pairs of plates 2 and the corrugatedfins 7, 9 are subsequently preheated to diffuse the Si in the claddingof the brazing sheet providing the plates 2 into the core to give an Sicontent of up to 1.65 mass % to the cladding. This preheating iseffected, with the preheating time and/or temperature for the usualbrazing operation altered. For example, the preheating time for theusual brazing operation is increased to 1.5 to 2 times when thepreheating time is altered. The preheating temperature for the usualbrazing operation is raised when the preheating temperature is altered.

The two preheated plates 2 in each pair are thereafter brazed to eachother along the peripheral edge portions thereof to form a flat hollowbody 5, and the corrugated fins 7, 9 are brazed to the flat hollowbodies 5.

The brazed assembly of the flat hollow bodies 5 and the corrugated fins7, 9 is then heated in an atmosphere containing a fluorinating gas toform a fluoride layer 23 over the inner and outer peripheral surfaces ofthe flat hollow bodies 5, i.e., the inner and outer surfaces of theplates 2, and the surfaces of the corrugated fins 7, 9. The fluorinatinggas is at least one gas selected from the group consisting of fluorinegas, chlorine trifluoride gas and nitrogen fluoride gas. Thefluorinating gas is diluted with an inert gas to prepare thefluorinating atmosphere. Preferably, this atmosphere has thefluorinating gas at a concentration of 5 to 80%. If the concentration ofthe fluorinating gas in the atmosphere is less than 5%, it is impossibleto form the fluoride layer 23 of required thickness, and it is difficultto obtain the desired corrosion resistance. The greater theconcentration, the higher the rate of formation of the fluoride layer23, whereas if the concentration exceeds 80%, the effect to increase therate of formation of the fluoride layer 23 levels off to render theincrease in the concentration unjustifiable and to result in the problemof an increased production cost. Accordingly, the fluorinating gasconcentration is preferably 5 to 80%, more preferably 10 to 60%.Although various inert gases are usable such as N₂ gas, Ar gas and Hegas, N₂ gas is especially preferable to use. For the fluorinationtreatment, the brazed assembly is held in the fluorinating atmospherepreferably at a temperature of at least 100° C. for at least 5 hours. Ifthe holding temperature is less than 100° C. or the holding time is lessthan 5 hours, it is difficult to effect diffusion into the surface layerportions of the inner and outer surfaces of the plates 2 and thesurfaces of the corrugated fins 7, 9, with the result that asatisfactory fluoride layer 23 becomes no longer available. Preferably,the holding temperature is at least 150° C. and the holding time is atleast 10 hours. The upper limit of the holding temperataure is nothigher than 600° C., and the upper limit of the holding time is notlonger than 50 hours. The pressure of the fluorinating atmosphere is notlimited specifically but can be set variously. Preferably, the pressureis in the range of 0.8×10⁵ to 1.5×10⁵ Pa.

In this way, the heat exchanger 1 is fabricated.

In the case where the heat exchanger 1 is used in a fuel cell system foruse in fuel cell motor vehicles or cogeneration systems as a COdiminishing heat exchanger to reduce the CO concentration of the fuelhydrogen gas produced by the reformer as previously described, acatalyst for selectively oxidizing CO is provided on the outerperipheral surfaces of the flat hollow bodies, i.e., on the outersurfaces of the plates 2 and the surfaces of the outer corrugated fins9.

FIG. 4 shows a modified plate for use as a heat exchanger componentproviding the flat hollow bodies 5 of the heat exchanger 1.

The plate 30 shown in FIG. 4 comprises a core layer 31 of pure aluminumor aluminum alloy, i.e., JIS A3003 alloy in this modification, two Al—Sialloy layers 32 covering respective opposite surfaces of the core layer31, an intermediate layer 33 of pure aluminum, i.e., JIS A1050, formedbetween each of the Al—Si alloy layers 32 and the core layer 31, and afluoride layer 34 formed in a surface layer portion of the Al—Si layer32. The material for the core layer 31 is not limited to JIS A3003alloy, nor is the material for the intermediate layer 33 limited to JISA1050.

A major portion of the entire Al—Si alloy layer 32 is up to 1.65 mass %in Si content as is the case with the plate 2 shown in FIG. 3 andalready described.

The intermediate layer 33 contains Si diffused thereinto from the Al—Sialloy layer 32. The diffused layer is indicated at 35. Preferably, thematerial JIS A1050 forming the intermediate layer 33 has added theretoZr and/or Mg in a total amount of 0.1 to 0.25 mass %. The addition of Zrand/or Mg enlarges the crystal grains of JIS A1050 forming the layer 33,with the result that Al-12 mass % Si eutectic is no longer produced alsoin the diffused layer 35 containing Si diffused thereinto from the alloylayer 32. However, if the total amount of the element or elements isless than 0.1 mass %, this effect is not available, whereas amounts inexcess of 0.25 mass % result in an increased cost. The total amount istherefore preferably 0.1 to 0.25 mass %. In view of the corrosionresistance and cost, the intermediate layer 33 has a thickness in aproportion of 5 to 25%, more preferably 15 to 25%, of the entirethickness taken as 100% of the plate 30.

The fluoride layer 34 comprises a fluoride produced by subjecting thesurface of the Al—Si alloy layer 32 of the plate 30 to a fluorinationtreatment. For the same reason as in the case of the plate 2 shown inFIG. 3, the fluoride layer is preferably 2 nm to 10 μm, more preferably20 nm to 3 μm, in thickness.

In the case where the heat exchanger 1 has flat hollow bodies 5comprising such plates 30 and is used in a fuel cell system, forexample, for use in fuel cell motor vehicles or cogeneration systems asa CO diminishing heat exchanger to reduce the CO concentration of thefuel hydrogen gas produced by the reformer, the Al—Si alloy layer 32 onthe inner side of the flat hollow bodies 5, i.e., on the inner side ofthe plates 30, need not always be up to 1.65 mass % in Si content, noris it necessary to provide the fluoride layer 34 in the surface layerportion thereof. Furthermore, the intermediate layer 33 need not beprovided on the inner side of the plates 30. The reason is that the heatexchanger encounters no problem with respect to corrosion resistanceeven if exposed to water or water containing a long-life coolant.

In the case where a fluid containing an acid component is passed throughboth the fluid channels 3 and the gas-phase fluid channels 8, it isrequired to give an Si content of up to 1.65 mass % to a major portionof the Al—Si alloy layer 32 on the inner and outer peripheral surfacesof the flat hollow bodies 5, i.e., the inner and outer surfaces of theplates 30, to provide an intermediate layer 33 between each Al—Si alloylayer 32 and the core layer 31, and to form a fluoride layer 34 in asurface layer portion of the Al—Si alloy layer.

The heat exchanger 1 comprising the plate 30 shown in FIG. 4 isfabricated by the process to be described below.

First, a five-layer brazing sheet is prepared which comprises a core ofpure aluminum or aluminum alloy, i.e., JIS A3003 alloy according to thepresent case, a cladding covering each of opposite sides of the core andmade of Al-7.5-12.5 mass % Si alloy brazing material, i.e., JIS A4004alloy brazing material in the present case, and an intermediate layer ofpure aluminum, i.e., JIS A1050 in the present case, provided between thecore and the cladding on each side of the core. Plates 30 are preparedfrom the brazing sheet by press work, each of the plates 30 having achannel-forming bulging portion 2 a and a header-forming bulging portion2 b bulging to a greater extent than the bulging portion 2 a andextending from each of opposite ends of the bulging portion 2 a.Preferably, the intermediate layer 33 of the brazing sheet has addedthereto Zr and/or Mg in a total amount of 0.1 to 0.25 mass %. Thecladding of the brazing sheet has a thickness in a proportion of 2 to25% of the entire thickness taken as 100% of the brazing sheet, and theintermediate layer 33 thereof has a thickness in a proportion of 5 to25%, preferably 15 to 25%, of the entire thickness taken as 100% of thebrazing sheet.

The plates 30 are then arranged in superposed pairs each comprising thecombination of two plates 30 with openings of the bulging portions 2 a(2 b ) of each type opposed to each other in corresponding relation sothat the outer surfaces of bottom walls of the header-forming bulgingportions 2 b of the adjacent pairs are in contact with each other. Outercorrugated fins 9 of bare pure aluminum or aluminum alloy, i.e., JISA3003 alloy in the present cases, are arranged between portionscorresponding to the channel-forming bulging portions 2 a of therespective adjacent pairs of plates 30, and inner corrugated fins 7 ofbare pure aluminum or aluminum alloy, i.e., JIS A3003 alloy in thepresent case, are arranged in the channel-forming bulging portions 2 aof the respective pairs of plates 30.

The two preheated plates 30 in each pair are thereafter brazed to eachother along the peripheral edge portions thereof to form a flat hollowbody 5, and the corrugated fins 7, 9 are brazed to the flat hollowbodies 5. For this brazing, the assembly of components is preheated inthe mode usual for brazing unlike the first process described.

The same fluorination treatment as in the first process described isthereafter performed to form a fluoride layer 34 over the inner andouter peripheral surfaces of the flat hollow bodies 5, i.e., the innerand outer surfaces of the plates 30, and the surfaces of the corrugatedfins 7, 9.

In this way, the heat exchanger 1 is fabricated.

In the case where the heat exchanger 1 is used in a fuel cell system foruse in fuel cell motor vehicles or cogeneration systems as a COdiminishing heat exchanger to reduce the CO concentration of the fuelhydrogen gas produced by a reformer as described above, a catalyst forselectively oxidizing CO is provided on the outer peripheral surfaces ofthe flat hollow bodies 5, i.e., on the outer surfaces of the plates 30and the surfaces of the outer corrugated fins 9 after the fluorinationtreatment.

In the case where the heat exchanger 1 is used in a fuel cell system foruse in fuel cell motor vehicles or cogeneration systems as a COdiminishing heat exchanger to reduce the CO concentration of the fuelhydrogen gas produced by a reformer as described above, a fluidcontaining no acid component, like the water containing a long-lifecoolant, flows through the fluid channels of the flat hollow bodies.Accordingly, a four-layer brazing sheet will be used which comprises acore of pure aluminum or aluminum alloy, i.e., JIS A3003 alloy accordingto the present case, a cladding covering each of opposite sides of thecore and made of Al-7.5-12.5 mass % Si alloy brazing material, i.e., JISA4004 alloy brazing material in the present case, and an intermediatelayer of pure aluminum, i.e., JIS A1050 in the present case, providedbetween the core and the cladding on one side of the core. In producingplates 30 each having a channel-forming bulging portion 2 a and aheader-forming bulging portion 2 b with a greater height than thebulging portion 2 a and extending from each of opposite ends of thebulging portion 2 a, the four-layer brazing sheet is subjected to presswork, with the intermediate layer existing side thereof positioned asthe outer side.

FIG. 5 shows another modified plate for use as a heat exchangercomponent of the flat hollow bodies 5 of the heat exchanger 1.

The plate 60 shown in FIG. 5 comprises a core layer 31 of pure-aluminumor aluminum alloy, i.e., JIS A3003 alloy in the present case, two Al—Sialloy layers 32 covering respective opposite surfaces of the core layer31, an intermediate layer 33 made of pure aluminum, i.e., JIS A1050 inthe present case, and formed between each of the Al—Si alloy layers 32and the core layer 31, an anodic oxide coating 61 formed over thesurface of each Al—Si alloy layer 32, a plating layer 62 containingnickel and formed over the surface of the anodic oxide coating 61, and afluoride layer 63 formed over the surface of the plating layer 62, theintermediate layer 33 containing Si diffused thereinto from the Al—Sialloy layer 32 to provide a diffused layer 35.

The core layer 31, two Al—Si alloy layers 32, intermediate layers 33 anddiffused layers 35 are the same as in the plate 30 shown in FIG. 4 andwill not be described repeatedly.

The plating layer 62 comprises, for example, an electroless nickelplating or electroless nickel-phosphorus alloy plating. The fluoridelayer 63 comprises a fluoride produced by subjecting the surface of theplating layer 62 to a fluorination treatment.

FIG. 6 shows another modified plate for use as a heat exchangercomponent of the flat hollow bodies 5 of the heat exchanger 1.

The plate 70 shown in FIG. 6 comprises a superposed layer group 73provided on the surface of the fluoride layer 34 of the plate 30 shownin FIG. 4 and comprising a nickel-containing plating layer 71, and afluoride layer 72 formed on the surface of the plating layer 71.

The plating layer 71 comprises, for example, an electroless nickelplating or electroless nickel-phosphorus alloy plating. The fluoridelayer 72 comprises a fluoride produced by subjecting the surface of theplating layer 71 to a fluorination treatment. Although the superposedlayer group 73 provided in the modification is one in number, this isnot limitative; at least two layer groups may be provided. For examplewhen two superposed layer group 73 are to be provided, anickel-containing plating layer is formed on the surface of the fluoridelayer 72 of the superposed layer group 73, and the surface of theplating layer is fluorinated to form on this surface a fluoride layercontaining a fluoride produced by the fluorination. Three or moresuperposed layer groups are also so formed as to provide a fluoridelayer 72 as the outermost layer.

In the case where the heat exchanger 1 has flat hollow bodies 5comprising such plates 60 or 70 shown in FIG. 5 or 6 and is used in afuel cell system, for example, for use in fuel cell motor vehicles orcogeneration systems as a CO diminishing heat exchanger to reduce the COconcentration of the fuel hydrogen gas produced by the reformer, theAl—Si alloy layer 32 on the inner side of the flat hollow bodies 5,i.e., on the inner side of the plates 60 or 70, need not always be up to1.65 mass % in Si content, nor is it necessary to provide the anodicoxide coating 61, plating layer 61 and fluoride layer 63, or thefluoride layer 34 and superposed layer group 73 on the surface of theAl—Si alloy layer 32. Furthermore, the intermediate layer 33 need not beprovided on the inner side of the plates 60 or 70. The reason is thatthe heat exchanger encounters no problem with respect to corrosionresistance even if exposed to water or water containing a long-lifecoolant.

In the case where a fluid containing an acid component is passed throughboth the fluid channels 3 and the gas-phase fluid channels 8, it isrequired to give an Si content of up to 1.65 mass % to a major portionof the Al—Si alloy layer 32 on the inner and outer peripheral surfacesof the flat hollow bodies 5, i.e., the inner and outer surfaces of theplates 60 or 70, to provide an intermediate layer 33 between each Al—Sialloy layer 32 and the core layer 31, and to form an anodic oxidecoating 61, plating layer 62 and fluoride layer 63, or a fluoride layer34 and superposed layer group 73 on the surface of the Al—Si alloylayer.

Given below are Experimental Examples wherein the advantage of theinvention was substantiated, and Comparative Experimental Examples.

Test pieces X, measuring 100 mm in length, 50 mm in width and 0.4 mm inthickness, were prepared from a brazing sheet comprising a core of JISA3003 alloy, two claddings of JIS A4004 alloy brazing material coveringrespective opposite sides of the core, and an intermediate layer formedbetween the core and one of the claddings and made of pure aluminumcontaining 0.15 mass % of Zr added thereto. The core, each of thecladdings and the intermediate layer of the test piece X had thicknessesof 54%, 13% and 20%, respectively, relative to the entire thickness ofthe test piece X taken as 100%. Test pieces Y, measuring 100 mm inlength, 50 mm in width and 0.5 mm in thickness, were prepared from abrazing sheet comprising a core of JIS A3003 alloy, and two claddings ofJIS A4004 alloy brazing material covering respective opposite sides ofthe core. The core and each of the claddings of the test piece Y hadthicknesses of 70% and 15%, respectively, relative to the entirethickness of the test piece Y taken as 100%.

EXPERIMENTAL EXAMPLE 1

A test piece X was placed into a vacuum heating furnace, heated at 575°C. for 40 minutes and thereafter heated at 612° C. for 12 minutes. Thetest piece X was then taken out of the furnace and placed into anatmospheric heating furnace, a mixture of fluorine gas and an inert gaswas introduced into the atmospheric heating furnace to provide afluorinating atmosphere in the interior of the furnace, and the testpiece X was heated at 400° C. for 24 hours for a fluorination treatment.The atmosphere had a fluorine gas concentration of 20%.

The test piece X was withdrawn from the furnace and checked for theappearance of a section thereof. FIG. 7 shows the result. With referenceto FIG. 7, indicated at 40 is the core, at 41 the cladding adjacent tothe intermediate layer, at 42 the cladding provided with no intermediatelayer, and at 43 the intermediate layer. FIG. 7 reveals the following.The cladding-41 adjacent to the intermediate layer 43 has a surfaceremaining almost free of any change and contains no Al-12 mass % Sieutectic, whereas the cladding 42 provided with no intermediate layerexhibits marked surface irregularities and contains Al-12 mass % Sieutectic. This result indicates that the Si contained in the cladding 41adjacent to the intermediate layer 43 diffused into the intermediatelayer 43. On the other hand, the Si in the cladding 42 provided with nointermediate layer 43 remained undiffused, and the fluorinationtreatment caused the portion of Al-12 mass % Si eutectic to produceSiH₄, which evaporated off. It is also seen that the Al-12 mass % Sieutectic remained.

EXPERIMENTAL EXAMPLE 2

A test piece X was treated under the same conditions as in ExperimentalExample 1 except that the holding temperature for the fluorinationtreatment was changed to 500° C.

When the test piece X was withdrawn from the atmospheric heating furnaceand checked, the test piece was found to have a white smooth surface onthe side thereof closer to the intermediate layer, while the test piecehad marked surface irregularities on the opposite side.

EXPERIMENTAL EXAMPLE 3

A test piece Y was placed into a vacuum heating furnace, heated at 575°C. for 40 minutes and thereafter heated at 612° C. for 12 minutes. Thetest piece Y was then taken out of the furnace temporarily, placed intothe vacuum heating furnace again, heated at 575° C. for 40 minutes andthereafter heated at 612° C. for 12 minutes. The test piece Y was thentaken out of the furnace and placed into an atmospheric heating furnace,and a mixture of fluorine gas and an inert gas was introduced into theatmospheric heating furnace to provide a fluorinating atmosphere in theinterior of the furnace. The atmosphere had a fluorine gas concentrationof 20%. The test piece Y was subsequently heated at 260° C. for 24hours.

When the test piece Y was withdrawn from the furnace and checked, thetest piece was found to have a white smooth surface.

EXPERIMENTAL EXAMPLE 4

A test piece Y was treated under the same conditions as in ExperimentalExample 3 except that the holding temperature for the fluorinationtreatment was changed to 400° C.

When the test piece Y was withdrawn from the atmospheric heating furnaceand checked, the test piece was found to have a brown smooth surface.

EXPERIMENTAL EXAMPLE 5

A test piece Y was treated under the same conditions as in ExperimentalExample 3 except that the holding temperature for the fluorinationtreatment was changed to 500° C.

When the test piece Y was withdrawn from the atmospheric heating furnaceand checked, the test piece was found to have a brown surface withminute irregularities.

COMPARATIVE EXPERIMENTAL EXAMPLE 1

A test piece Y was placed into a vacuum heating furnace, heated at 575°C. for 40 minutes and thereafter heated at 612° C. for 12 minutes. Thetest piece Y was then taken out of the furnace and placed into anatmospheric heating furnace, a mixture of fluorine gas and an inert gaswas introduced into the atmospheric heating furnace to provide afluorinating atmosphere in the interior of the furnace, and the testpiece Y was heated at 400° C. for 24 hours for a fluorination treatment.The atmosphere had a fluorine gas concentration of 20%.

When the test piece Y was withdrawn from the furnace and checked, thetest piece was found to have marked surface irregularities.

COMPARATIVE EXPERIMENTAL EXAMPLE 2

A test piece Y was treated under the same conditions as in ComparativeExperimental Example 1 except that the holding temperature for thefluorination treatment was changed to 500° C.

When the test piece Y was withdrawn from the atmospheric heating furnaceand checked, the test piece was found to have marked surfaceirregularities.

EXPERIMENTAL EXAMPLE 6

Test pieces X were treated under the same conditions as in ExperimentalExample 1. A corrosive aqueous solution was prepared which was 3 in pHand contained 10 ppm of hydrochloric acid, 50 ppm of nitric acid, 1000ppm of formic acid and 300 ppm of acetic acid. Each of the test pieces Xwas heated at 50° C. for 15 minutes, then cooled in the air for 4minutes, and thereafter immersed in the corrosive solution for 1 minuteas a simulated corrosion cycle, and this cycle was repeated 2500 timesfor a corrosion test. The test piece X was checked for a reduction inthe thickness thereof every time a predetermined number of corrosioncycles were completed. FIG. 8 shows the variations in the reduction ofthickness of the test pieces.

EXPERIMENTAL EXAMPLE 7

Test pieces Y were treated under the same conditions as in ExperimentalExample 4, subjected to the same corrosion test as in ExperimentalExample 6, and checked for a reduction in thickness every time apredetermined number of simulated corrosion cycles were completed. FIG.8 shows the variations in the reduction of thickness.

COMPARATIVE EXPERIMENTAL EXAMPLE 3

Test pieces Y were placed into a vacuum heating furnace, heated at 575°C. for 40 minutes and thereafter heated at 612° C. for 12 minutes. Thetest pieces Y were then taken out of the furnace and placed into anatmospheric heating furnace, and a mixture of fluorine gas and an inertgas was introduced into the atmospheric heating furnace to provide afluorinating atmosphere in the interior of the furnace. The atmospherehad a fluorine gas concentration of 20%. The test pieces Y weresubsequently heated at 260° C. for 24 hours.

The test pieces Y were subjected to the same corrosion test as inExperimental Example 6, and checked for a reduction in thickness everytime a predetermined number of simulated corrosion cycles werecompleted. FIG. 8 shows the variations in the reduction of thickness.

COMPARATIVE EXPERIMENTAL EXAMPLE 4

Test pieces made of JIS SUS304 and measuring 100 mm in length, 50 mm inwidth and 0.9 mm in thickness were subjected to the same corrosion testas in Experimental Example 6 and checked for a reduction in thicknessevery time a predetermined number of simulated corrosion cycles werecompleted. FIG. 8 shows the variations in the reduction of thickness.

COMPARATIVE EXPERIMENTAL EXAMPLE 5

Test pieces made of JIS A3003 alloy and measuring 100 mm in length, 50mm in width and 0.9 mm in thickness were placed into an atmosphericheating furnace, and a mixture of fluorine gas and an inert gas wasintroduced into the furnace to provide a fluorinating atmosphere in theinterior of the furnace. The atmosphere had a fluorine gas concentrationof 20%. The test pieces were then heated at 400° C. for 24 hours,thereafter subjected to the same corrosion test as in ExperimentalExample 6 and checked for a reduction in thickness every time apredetermined number of simulated corrosion cycles were completed. FIG.8 shows the variations in the reduction of thickness.

The results shown in FIG. 8 reveal that the reductions in the thicknessof test pieces due to corrosion and determined in Experimental Examples6 and 7 are comparable to those of stainless steel, and that ComparativeExperimental Example 3 results in marked reductions in thickness. Itappears that the test pieces of Comparative Experimental Example 5 weresubjected to the same conditions as the outer corrugated fins of heatexchangers. The result shown therefore substantiates that even when theheat exchanger of the invention is used in the fuel cell system for fuelcell motor vehicles or cogeneration systems for reducing the COconcentration of the fuel hydrogen gas produced by the reformer, theouter corrugated fins are not susceptible to corrosion.

EXPERIMENTAL EXAMPLE 8

Test piece X were treated under the same conditions as in ExperimentalExample 2, then subjected to a corrosion test under the same conditionsas in Experimental Example 6, and checked for the appearance of asection thereof. FIG. 9 shows the result. With reference to FIG. 9,indicated at 50 is the core, at 51 the cladding adjacent to theintermediate layer, at 52 the cladding provided with no intermediatelayer, and at 53 the intermediate layer. FIG. 9 reveals the following.The cladding 51 adjacent to the intermediate layer 53 has a surfaceremaining almost free of any change and contains no Al-12 mass % Sieutectic, whereas the cladding 52 provided with no intermediate layerexhibits marked surface irregularities, and corrosion develops fromAl-12 mass % Si eutectic in this cladding 52 to the core 50.

The plates 2, 30, i.e., heat exchanger components of the presentinvention, have high corrosion resistance not only to fluids containingan acid component but also to those containing an alkaline component.

Other embodiments of the present invention include pure aluminum oraluminum alloy products comprising a component which is similar inconstruction to the plates 2 and 30 described above, for example, acomponent in the form of a flat plate, bent plate or tube. With suchpure aluminum or aluminum alloy products, at least one side or surfaceof the component is exposed to a fluid containing an acid component oralkaline component.

INDUSTRIAL APPLICABILITY

The heat exchangers of the present invention are suitable for use infuel cell systems useful, for example, for fuel cell motor vehicles orcogeneration systems for reducing the CO concentration of fuel gas(hydrogen gas)produced by a reformer.

1. A heat exchanger comprising a heat exchanger component having asurface covered with an Al—Si alloy layer, the Al—Si alloy layer havinga fluoride layer formed in a surface layer portion thereof, the Al—Sialloy layer of the heat exchanger component having a portion up to 1.65mass % in Si content.
 2. A heat exchanger according to claim 1 whereinthe fluoride layer is 2 nm to 10 μm in thickness.
 3. A heat exchangeraccording to claim 1 wherein the fluoride layer comprises a fluorideproduced by subjecting a surface of the Al—Si alloy layer of the heatexchanger component to a fluorination treatment.
 4. A heat exchangeraccording to claim 1 wherein an anodic oxide coating is formed over asurface of the Al—Si alloy layer of the heat exchanger component, and aplating layer containing nickel is formed over a surface of the anodicoxide coating, the fluoride layer being formed on a surface of theplating layer and comprising a fluoride produced by subjecting thesurface of the plating layer to a fluorination treatment.
 5. A heatexchanger according to claim 1 wherein the fluoride layer is providedover a surface thereof with at least one superposed layer groupcomprising a plating layer containing nickel and a fluoride layercomprising a fluoride produced by subjecting a surface of the platinglayer to a fluorination treatment.
 6. A heat exchanger according toclaim 1 wherein the heat exchanger component comprises a core layer ofpure aluminum or aluminum alloy, and an Al—Si alloy layer covering eachof opposite surfaces of the core layer, the core layer containing Sidiffused thereinto from the Al—Si alloy layer, the Al—Si alloy layerhaving a portion up to 1.65 mass % in Si content.
 7. A heat exchangeraccording to claim 6 wherein at least one surface of the heat exchangercomponent is exposed to a fluid containing an acid component.
 8. A heatexchanger according to claim 1 wherein the heat exchange component has aportion comprising a core layer of pure aluminum or aluminum alloy, andtwo Al—Si alloy layers covering respective opposite surfaces of the corelayer, and an intermediate layer of pure aluminum is formed between oneof the Al—Si alloy layers and the core layer, the intermediate layercontaining Si diffused thereinto from the Al—Si alloy layer, the Al—Sialloy layer adjacent to the intermediate layer having a portion up to1.65 mass % in Si content.
 9. A heat exchanger according to claim 1wherein the heat exchange component comprises a core layer of purealuminum or aluminum alloy, and two Al—Si alloy layers coveringrespective opposite surfaces of the core layer, and an intermediatelayer of pure aluminum is formed between each of the Al—Si alloy layersand the core layer, the intermediate layer containing Si diffusedthereinto from the Al—Si alloy layer, the Al—Si alloy layer having aportion up to 1.65 mass % in Si content.
 10. A heat exchanger accordingto claim 8 or 9 wherein the pure aluminum making the intermediate layerhas added thereto Zr and/or Mg in a total amount of 0.1 to 0.25 mass %.11. A heat exchanger according to claim 8 or 9 wherein the intermediatelayer has a thickness in a proportion of 5 to 25% of the entirethickness taken as 100% of the heat exchanger component.
 12. A heatexchanger according to claim 8 or 9 wherein a surface of the heatexchanger component on the side thereof where the intermediate layerexists is exposed to a fluid containing an acid component.
 13. A heatexchanger according to claim 1 which comprises a plurality of parallelhollow bodies each having a fluid channel inside thereof and finsarranged between and brazed to respective pairs of adjacent flat hollowbodies, the heat exchanger component being each of the flat hollowbodies.
 14. A heat exchanger according to claim 1 which comprises aplurality of parallel hollow bodies each having a fluid channel insidethereof and fins arranged between and brazed to respective pairs ofadjacent flat hollow bodies, each of the hollow bodies comprising twoplates brazed to each other at peripheral edge portions thereof, the twoplates defining therebetween a bulging fluid channel and a bulgingheader-forming portion communicating with each of opposite ends of thefluid channel, the heat exchanger component being each of the plates.15. A heat exchanger according to claim 13 or 14 wherein a fluidcontaining an acid component flows through at least one of the fluidchannel inside each of the flat hollow bodies and a clearance betweeneach pair of adjacent flat hollow bodies.
 16. A heat exchanger accordingto claim 13 or 14 wherein fuel hydrogen gas produced by reforming in afuel cell system flows through a clearance between each pair of adjacentflat hollow bodies, and an outer peripheral surface of each of the flathollow bodies is covered with an Al—Si alloy layer, a fluoride layerbeing formed in a surface layer portion of the Al—Si alloy layer, theAl—Si alloy layer having a portion up to 1.65 mass % in Si content, acatalyst for selectively oxidizing CO being provided on the outerperipheral surface of each of the flat hollow bodies and on a surface ofeach of the fins, the catalyst being serviceable to diminish CO in thefuel hydrogen gas.
 17. A fuel cell system comprising a heat exchangeraccording to any one of claims 1 to 16 for diminishing Co.
 18. A fuelcell motor vehicle having installed therein a fuel cell system accordingto claim
 17. 19. A cogeneration system comprising a fuel cell systemaccording to claim
 17. 20. A process for fabricating a heat exchangercharacterized by making plates each having a channel-forming bulgingportion and a header-forming bulging portion bulging to a greater extentthan the bulging portion and extending from each of opposite ends of thechannel-forming bulging portion, from a brazing sheet comprising a coreof pure aluminum or aluminum alloy and a cladding of Al-7.5-12.5 wt. %Si alloy brazing material covering each of opposite sides of the core,arranging the plates in superposed pairs each comprising the combinationof two plates with openings of the bulging portions of each type opposedto each other in corresponding relation so that outer surfaces of bottomwalls of the header-forming bulging portions of the adjacent pairs arein contact with each other and arranging fins of bare pure aluminum oraluminum alloy between portions corresponding to the channel-formingbulging portions of the respective adjacent pairs of plates, preheatingthe resulting combination of the pairs of plates and the fins to diffusethe Si in the cladding of the brazing sheet providing the plates intothe core, brazing the two preheated plates in each pair to each otheralong the peripheral edge portions thereof to form a flat hollow body,brazing the fins to the respective adjacent pairs of flat hollow bodies,and heating the brazed assembly of the flat hollow bodies and the finsin an atmosphere containing a fluorinating gas to form a fluoride layerover surfaces of the flat hollow bodies and surfaces of the fins.
 21. Aprocess for fabricating a heat exchanger characterized by making plateseach having a channel-forming bulging portion and a header-formingbulging portion bulging to a greater extent than the bulging portion andextending from each of opposite ends of the channel-forming bulgingportion, from a brazing sheet comprising a core of pure aluminum oraluminum alloy, a cladding of Al-7.5-12.5 wt. % Si alloy brazingmaterial covering each of opposite sides of the core, and anintermediate layer of pure aluminum formed between the core and thecladding over at least one of the opposite sides thereof, arranging theplates in superposed pairs each-comprising the combination of two plateswith openings of the bulging portions of each type opposed to each otherin corresponding relation so that outer surfaces of bottom walls of theheader-forming bulging portions of the adjacent pairs are in contactwith each other and arranging fins of bare pure aluminum or aluminumalloy between portions corresponding to the channel-forming bulgingportions of the respective adjacent pairs of plates, preheating theresulting combination of the pairs of plates and the fins to diffuse theSi in the cladding of the brazing sheet providing the plates into thecore, brazing the two preheated plates in each pair to each other alongthe peripheral edge portions thereof to form a flat hollow body, brazingthe fins to the respective adjacent pairs of flat hollow bodies, andheating the brazed assembly of the flat hollow bodies and the fins in anatmosphere containing a fluorinating gas to form a fluoride layer over asurface of each of the flat hollow bodies on the core side thereof wherethe intermediate layer exists and over surfaces of the fins.
 22. Aprocess for fabricating a heat exchanger according to claim 21 whereinthe pure aluminum providing the intermediate layer of the brazing sheetmaking the plates has added thereto Zr and/or Mg in a total amount of0.1 to 0.25 mass %.
 23. A process for fabricating a heat exchangeraccording to claim 21 wherein the intermediate layer of the brazingsheet providing the plates has a thickness in a proportion of 5 to 25%of the entire thickness taken as 100% of the brazing sheet.
 24. Aprocess for fabricating a heat exchanger according to claim 20 or 21wherein the core of the brazing sheet providing the plates and the finsare each made of JIS A3003 alloy.
 25. A process for fabricating a heatexchanger according to claim 20 or 21 wherein the cladding of thebrazing sheet providing the plates has a thickness in a proportion of 2to 25% of the entire thickness taken as 100% of the brazing sheet.
 26. Aprocess for fabricating a heat exchanger according to claim 20 or 21wherein the fluorinating gas is at least one gas selected from the groupconsisting of fluorine gas, chlorine trifluoride gas and nitrogenfluoride gas, and the fluorinating gas is diluted with an inert gas toprepare the atmosphere.
 27. A process for fabricating a heat exchangeraccording to claim 26 wherein the atmosphere contains the fluorinatinggas at a concentration of 5 to 80%.
 28. A process for fabricating a heatexchanger according to claim 26 wherein the atmosphere contains thefluorinating gas at a concentration of 10 to 60%.
 29. A process forfabricating a heat exchanger according to claim 20 or 21 wherein acatalyst for selectively oxidizing CO is provided on outer peripheralsurfaces of the flat hollow bodies and on surfaces of the fins after thefluoride layer is formed.
 30. A product of pure aluminum or aluminumalloy comprising a component having a surface covered with an Al—Sialloy layer, the Al—Si alloy layer having a fluoride layer formed on asurface layer portion thereof, the Al—Si alloy layer of the componenthaving a portion up to 1.65 mass % in Si content.
 31. A product of purealuminum or aluminum alloy according to claim 30 wherein the fluoridelayer is 2 nm to 10 μm in thickness.
 32. A product of pure aluminum oraluminum alloy according to claim 30 wherein the fluoride layercomprises a fluoride produced by subjecting a surface of the Al—Si alloylayer of the component to a fluorination treatment.
 33. A product ofpure aluminum or aluminum alloy according to claim 30 wherein an anodicoxide coating is formed over a surface of the Al—Si alloy layer of thecomponent, and a plating layer containing nickel is formed over asurface of the anodic oxide coating, the fluoride layer being formedover a surface of the plating layer and comprising a fluoride producedby subjecting the surface of the plating layer to a fluorinationtreatment.
 34. A product of pure aluminum or aluminum alloy according toclaim 30 wherein the fluoride layer is provided over a surface thereofwith at least one superposed layer group comprising a plating layercontaining nickel and a fluoride layer comprising a fluoride produced bysubjecting a surface of the plating layer to a fluorination treatment.35. A product of pure aluminum or aluminum alloy according to claim 30wherein the component comprises a core layer of pure aluminum oraluminum alloy, and an Al—Si alloy layer covering each of oppositesurfaces of the core layer, the core layer containing Si diffusedthereinto from the Al—Si alloy layer, the Al—Si alloy layer having aportion up to 1.65 mass % in Si content.
 36. A product of pure aluminumor aluminum alloy according to claim 35 wherein at least one surface ofthe component is exposed to a fluid containing an acid component oralkaline component.
 37. A product of pure aluminum or aluminum alloyaccording to claim 30 wherein the component has a portion comprising acore layer of pure aluminum or aluminum alloy, and two Al—Si alloylayers covering respective opposite surfaces of the core layer, and anintermediate layer of pure aluminum is formed between one of the Al—Sialloy layers and the core layer, the intermediate layer containing Sidiffused thereinto from the Al—Si alloy layer, the Al—Si alloy layeradjacent to the intermediate layer having a portion up to 1.65 mass % inSi content.
 38. A product of pure aluminum or aluminum alloy accordingto claim 30 wherein the component comprises a core layer of purealuminum or aluminum alloy, and two Al—Si alloy layers coveringrespective opposite surfaces of the core layer, and an intermediatelayer of pure aluminum is formed between each of the Al—Si alloy layersand the core layer, the intermediate layer containing Si diffusedthereinto from the Al—Si alloy layer, the Al—Si alloy layer having aportion up to 1.65 mass % in Si content.
 39. A product of pure aluminumor aluminum alloy according to claim 37 or 38 wherein the pure aluminummaking the intermediate layer has added thereto Zr and/or Mg in a totalamount of 0.1 to 0.25 mass %.
 40. A product of pure aluminum or aluminumalloy according to claim 37 or 38 wherein the intermediate layer has athickness in a proportion of 5 to 25% of the entire thickness taken as100% of the component.
 41. A product of pure aluminum or aluminum alloyaccording to claim 37 or 38 wherein a surface of the component on theside thereof where the intermediate layer exists is exposed to a fluidcontaining an acid component or alkaline component.